388 On Light. 



appear in succession, whenever I am surrounded by 

 darkness, whether in a dark room in the day time, or in 

 the darkness of the night. Their existence I attribute 

 to the vascular circulation. This, if I conjecture right, 

 occasions in my eyes an unnatural, and excessive, pres- 

 sure upon the optic nerve. 



Whenever my eyes feel crowded, (a sensation, per- 

 fectly distinguishable, particularly from the frequent rep- 

 etition of it at intervals, in some degree regular;) around 

 the centre of vision JI ashes of light, distantly resembling 

 what is called a glory ^ but with little regularity of form, 

 take the place of these luminous points j and succeed 

 each other with a considerable degree of rapidity. 

 When 1 am in bed, and lie upon my back ; they become 

 intense, and vivid. If I turn upon my side, the brill- 

 iancy begins immediately to lessen, and after some time 

 fades chiefly away. These appearances I attribute to 

 an increased pressure, in both cases, upon the optic 

 nerve. This pressure I suppose to be greatest, when I 

 lie upon my back, and to produce the peculiar splendour 

 of the flashes, at that time. When I lie upon my side, 

 and my eyes are in a horizontal position ; 1 suppose the 

 pressure to be less, and the splendour for this reason to 

 be diminished. 



In many instances also, when my eyes are less crowd- 

 ed, luminous clouds; (phraseology, which better accords 

 with the appearances, than any other, that I can think 

 of;) commencing near the exteriour limit of vision, and 

 irregular in their form, but filling up almost the whole 

 field, contract themselves with considerable rapidity to- 

 ward the centre, and there vanish. Their colour, to- 

 wards the exteriour limit, is very frequently a reddish 

 browm. As they advance inward, it becomes a strong 

 Saxon blue ; and in the end a green, which is often vivid. 

 Of these clouds, floating inward, lessening, and vanish- 

 ing, there is in such cases, an uninterrupted succession. 

 Their luminous appearance I attribute to an unnatural 

 degree of pressure. Of their figure, and motion, I am 

 unable to assign the causes. 



Whenever it is dark ; if I turn my eyes suddenly to 

 the right, and left ; on the side of each, opposite to thatj 

 towards which I turn them; there is formed a sudden, 



